The use of non-rising stem gate valves to control the flow of high pressure fluid or gas is well known in the energy producing industry. It is also known to use a metal-to-metal backseat seal between the valve stem and the valve bonnet to seal the inner valve chamber from the exterior environment in order to allow repacking of the valve stem or replacement of the valve stem journal bearings. See for example U.S. Pat. No. 4,149,558 "Selective Backseat Valve", By McGee et al.
In the oil and gas producing industries, this valve has been further modified so as to automatically backseat under high temperature conditions, such as a fire, during which the resilient packing seal between the valve stem and the valve bonnet may be damaged and allow leakage. This modification, disclosed in U.S. Pat. No. 4,289,157 "Valve With Heat-Responsive Bearing Assembly Providing Backseat Arrangement", by McGee, includes a fusible washer-like annulus placed in the valve stem journal bearing assembly. Upon heating of the valve to an abnormally high temperature, the fusible annulus melts and runs out of a passageway provided to the exterior of the valve. The valve stem is now free to rise a limited extent due to the internal valve chamber pressure, thereby engaging the annular metal-to-metal sealing surfaces provided on the valve bonnet and the valve stem.
While this design is entirely satisfactory, it does not meet new standards for fire resistant valves for use in petroleum or natural gas applications. In particular, these standards require that the metal-to-metal backseat be established and maintained even under conditions of zero gauge pressure within the valve chamber. It will be appreciated by those skilled in the art that the apparatus of U.S. Pat. No. 4,289,157 requires a finite pressure differential between the interior chamber and the valve exterior in order to drive the valve stem upwards and effect the backseat seal.
What is needed is a simple, effective means for engaging the metal-to-metal backseat seal in a non-rising stem valve under the combined conditions of abnormally high valve temperature and zero internal valve chamber gauge pressure.